An examination of the content and processes of evidence-based programs is critical for empirically evaluating theories about how programs work, the "action theory" of the program (West et al. in American Journal of Community Psychology, 21, 571-605, 1993). The New Beginnings Program (NBP; Wolchik et al., 2007), a parenting-after-divorce preventive intervention, theorizes that program-induced improvements in parenting across three domains: positive relationship quality, effective discipline, and protecting children from interparental conflict, will reduce the negative outcomes that are common among children from divorced families. The process theory is that home practice of program skills related to these parenting domains is the primary mechanism leading to positive change in parenting. This theory was tested using multi-rater data from 477 parents in the intervention condition of an effectiveness trial of the NBP (Sandler et al. 2016a, 2016b). Four research questions were addressed: Does home practice of skills predict change in the associated parenting outcomes targeted by the program? Is the effect above and beyond the influence of attendance at program sessions? What indicators of home practice (i.e., attempts, fidelity, efficacy, and competence) are most predictive of improvements in parenting? Do these indicators predict parenting improvements in underserved subpopulations (i.e., fathers and Latinos)? Structural Equation Modeling analyses indicated that parent-reported efficacy and provider-rated parent competence of home practice predicted improvements in the targeted parenting domains according to both parent and child reports. Moreover, indicators of home practice predicted improvements in parenting for fathers and Latinos, although patterns of effects varied by parenting outcome.